The video below is a demo of a few simple methods of protecting your head against strikes:

The most common strike to the head is the frontal jab, and it is the most difficult one to protect against. The first thing for the defender to remember is that the best escape is by movement. Typically, when we get hit in the face, we tend to get shocked and freeze in our place. To prevent that and learn to move right away, practice stepping away from the strike with your whole body, so that on contact your foot moves back and the weight of the body is transferred in the same direction. The demo above shows a nice trick - lowering your chin - which makes stepping back much easier and reduces the impact of the strike.

Another static defense method involves turning your head to make the strike slide off or tilting your head to almost lay it over the incoming fist (this technique is sometimes called a "pillow" because the head ends up resting on top of the incoming arm as it passes under the chin.

An important thing to remember in all of these is to return your head to original position after each escape to get ready for the next strike.

The most advanced type of defense is sometimes referred to as "combing" and involves defender's hands sliding over the head back to front or front to back as if coming the hair - hence the name. This method works very well in a multiple opponent or crowded situation, especially if you have little room to maneuver. It makes use of the elbows and forearms to simultaneously protect your head and strike or push around the attackers.